The invention relates to a magnetic multilayer sensor. The sensor may present a spin tunneling or a magnetoreresistive effect. Magnetoresistive sensors are known from U.S. Pat. No. 5,686,837.
Certain magnetoresistive sensors use the so-called GMR effect.
Giant magnetoresistance (GMR) is the phenomenon that the resistance of a material (e.g. a magnetic multilayer) depends on the angle between magnetization directions (e.g. of different layers). Examples of GMR material systems are exchange-biased spin valves and multilayers comprising artificial antiferromagnets (AAF""s).
For automotive applications it is required that magnetic sensors also operate at high temperatures (up to 175xc2x0 C.-200xc2x0 C.). Moreover, the sensors should operate in a broad field range and may not be irreversibly affected in an even larger field range. At present no material system is available that can fulfil both of these requirements and has suitable magnetoresistance characteristics for practical sensors.
For example, the antiferromagnetically coupled multilayers of commercially available sensors could fulfil the requirements reasonably; however, the symmetric output curve complicates (e.g. with respect to full Wheatstone-bridge configurations) or even inhibits (e.g. analog angle sensor in saturation) many applications.
Exchange-biased spin valves show poor thermal stability or have a too small field range. On the other hand, AAF""s are only stable up to limited magnetic fields. At higher magnetic fields the output characteristics can even be flipped, which is unacceptable because of safety matters. Therefore a different, robust method of fixing a magnetization direction is needed.
In order to eliminate output variations due to temperature changes often (Wheatstone-)bridge configurations are used. This requires MR-elements with opposite (signs of) output signals, which in principle could be accomplished if elements with opposite directions of the fixed magnetization could be made.
Thus it is an object of the present invention, in order to enable practical applications of GMR in field sensors, to provide a new material system that provides an unambiguous, asymmetric output signal in a broad temperature and magnetic field range.
The above object is met by a sensor which comprises a substrate which carries a free and a pinned ferromagnetic layer for presenting the magnetoresistive effect, said pinned layer comprising an artificial antiferromagnet layer system (AAF), and an exchange biasing layer of an IrMn type material, said exchange biasing layer being adjacent to and in contact with the AAF layer system.
By this multilayer system the following is achieved:
the blocking temperature of IrMn is higher than that of FeMn and unlike NiMn no annealing treatment is needed. The blocking temperature is the temperature above which the exchange biasing between the antiferromagnetic IrMn type layer and the pinned layer vanishes (reduces to zero).
the AAF diminishes magnetostatic coupling between the pinned and the free layer;
the AAF provides a large rigidity in magnetic fields because of the very small net magnetic moment (theoretically zero in the ideal case), however it is stable in two opposite directions;
in combination with the exchange-biasing layer this provides a very large magnetic field range (for comparison: in exchange-biased spin valves the exchange biasing field is typically at most 20-30 kA/m);
by means of exchange-biasing an unambiguous curve is obtained;
the exchange-biasing provides a way to set the direction of the pinned layers, which is required e.g. for Wheatstone-bridge configurations.
The AAF may comprise a Co/non-magnetic metal/Co system, but preferably CoFe/non-magnetic metal/CoFe is used, because in such systems an anisotropy can be induced.
The non-magnetic metal in the AAF preferably is Ru, which provides strong coupling and appears to be very stable (no oxidation, no diffusion) which is very important for the definition of the thinnest and most critical layer in the AAF stack.
To eliminate diffusion Ni preferably is avoided at all interfaces with Cu: CoFe is used instead of NiFe. Moreover, this increases the GMR ratio. Compared with Co, CoFe gives a lower coercivity (and better texture) in the free layer.
The exchange-biasing field is larger in inverted than in conventional spin valves; therefore the exchange-biasing layer is preferably positioned nearest to the substrate. However, in this case a buffer layer may be needed to obtain the required texture. Investigations showed that (2 nm) NiFe on (for example 3.5 nm) Ta is preferred.
These and other embodiments of the invention will be elucidated with reference to the drawing.